In recent years, with the diffusion of optical fiber transmission, various techniques have been required to realize optical signal processing functions. Furthermore, a technique has been also required to integrate a great number of optical elements with a high density. Among the techniques as described above, a planar lightwave circuit (hereinafter referred to as PLC) is known. The PLC is generally an optical circuit that is structured so that an optical waveguide composed of a core and a clad for propagating an optical signal and optical function elements are integrated on a silicon substrate or a quartz substrate. The PLC is superior in productivity and reliability and is also superior in integration and high function. Thus, the development for an optical module which integrates, for example, PLC, light-emitting elements such as LD, and light-receiving elements such as PD in a high quantity at a high density has been actively carried out.
FIG. 1 illustrates the configuration of a conventional optical module. An optical module 1 is structured so that a housing 3 includes therein a printed circuit board 20 having thereon a PLC 30, PD packages 40, and connectors 22. As an example, the PLC 30 has, as an optical function element, an arrayed waveguide grating-type optical multiplexer/demultiplexer (hereinafter referred to as AWG) (see Non-patent Publication 1 for example). The AWG is composed of: an input slab waveguide 34 coupled to an input waveguide 33 connected to optical fiber 2, an output slab waveguide 36 coupled to an output waveguide 37, and an array waveguide 35 connecting the input slab waveguide 34 to the output slab waveguide 36.
An end face of the output waveguide 37 of the AWG (i.e., an end face of the PLC 30) is joined to the PD packages 40. The optical module 1 is an optical power monitor in which a wavelength division multiplexing signal input from the optical fiber 2 is branched by the AWG to optical signals having individual wavelengths and the signals are received by the individual light-receiving faces of the PD stored in the PD packages 40.
FIG. 2 illustrates the configuration of a conventional PD package. In the PD package 40, a PD array 43 having a plurality of light-receiving faces 44 (i.e., a plurality of optical elements) are stored in a ceramic housing 41. The ceramic housing 41 is connected to a glass cover 42 by soldering, so that the PD array 43 is sealed in an air-tight manner. The PD array 43 is connected, via an electric wiring 45 electrically connected thereto, to other elements exterior to the housing 41. The PD package 40 is called a chip scale package-type PD array. The PD package 40 is much smaller when compared to a PD array module in which a plurality of PDs of a CAN package are arranged. Thus, the PD package 40 attracts attention as a technique by which many PDs can be integrated with a low cost (see Patent Publication 1 for example).
FIG. 3 illustrates a method of fixing a PLC and PD packages in a conventional optical module. FIG. 3 is a cross-sectional view taken along III-III in FIG. 1. The PLC 30 is structured so that a substrate 32 has thereon an optical waveguide layer 31. The optical waveguide layer 31 has AWG. The output waveguides 37a and 37b formed in AWG have vertically-polished end faces that are joined to the glass covers 42 of the PD packages 40 by UV adhesive agent. The respective light-receiving faces of the PD array 43 are optically connected to the respective output waveguides 37.
The electric wiring 45, which is electrically connected to the PD array 43, is drawn to the exterior of the housing 41 and is attached with a lead pin 46. The electric wiring 45 or the lead pin 46 is soldered to an electric wiring 21 formed on the printed circuit board 20 and is electrically connected to the connectors 22a and 22b. In this way, the PD packages 40 are fixed to the surface of the printed circuit board 20 and have a function to fix the PLC 30 on the printed circuit board 20.
The conventional optical module 1 had a disadvantage in that a temperature fluctuation tends to cause the fluctuation of a characteristic such as a light-receiving sensitivity characteristic. Generally, the PLC 30 formed on the substrate 32, the printed circuit board 20, and the housing 3 have different thermal expansion coefficients. Thus, when the optical module 1 has a temperature fluctuation, each size of the substrate 32, the printed circuit board 20 and the housing 3 varies respectively. Due to variation of sizes, a high stress is applied to the PD packages 40 having a function to fix the PLC 30 on the printed circuit board 20 or the housing 3. This stress causes positional displacement at the face at which the PLC 30 is connected to the PD packages 40. This positional displacement causes a fluctuation of the coupling efficiency between the light-receiving face 44 of the PD array 43 and the optical output end of the output waveguide 37 of the PLC 30.
It is an objective of the present invention to provide an optical module for which the optical module is structured to mitigate the stress applied to the package to thereby reduce thermal fluctuations in characteristics.    Patent Publication 1: Japanese Laid-Open Publication No. 2006-128514    Non-patent Publication 1: T. Oyama et. al, “40-ch optical power channel monitor module using AWG and CSP-PD array,” 2006, The Institute of Electronics, Information and Communication Engineers, Electronics Society Convention Lecture Paper Collection 1, C-3-78, p. 200